Enantioselective Friedel‐Crafts Alkylation of Indoles with α, β‐Unsaturated Aldehydes Catalyzed by Recyclable Second‐generation MacMillan Catalysts

Abstract

Imidazolium ionic liquids containing second-generation MacMillan catalysts were synthesized and evaluated as organocatalysts (10 mol%) for enantioselective Friedel-Crafts reaction between N-benzylindole and crotonaldehyde using trifluoroacetic acid as a co-catalyst (10 mol%) at À 60 °C, the corresponding product was obtained in 87 % yield with 89 % ee. The scope and limitation of organocatalyst were studied using substituted indoles and different α, β-unsaturated aldehydes, and enantioselective alkylated indoles were obtained in 43-95 % yields with 58-90 % ee's. The second-generation modified MacMillan catalyst 6 was successfully recovered and reused up to four cycles and a significant drop in yields and ee was observed after the third recycle.